This invention relates to a charge-coupled device having a semiconductor body comprising a semiconductor layer of a first conductivity type adjoining a surface, means for depleting the semiconductor layer throughout its thickness at least in the absence of the supply of majority charge carriers while avoiding breakdown, a sequence of transport electrodes on the surface above the semiconductor layer, which are separated from this layer by a blocking layer and are connected to clock voltage sources to form in the semiconductor layer potential wells separated from the surface for storing and transporting information-carrying charge packets, an input stage comprising a supply zone for supplying said majority charge carriers and an input electrode, which is located between the supply zone and the transport electrodes, is separated from the semiconductor surface by an isolating layer and by which in the semiconductor layer a potential well can be induced with an input signal source for applying a voltage difference between the supply zone and the input electrode, which determines the value of a charge carrier packet flowing from the supply zone into the potential well under the input electrode, while in order to obtain a practically linear relation between the value of the said charge carrier packet and the input signal, means are provided by which a potential well can be obtained under the input electrode whose minimum is located closer to the surface than in the potential wells induced under the transport electrodes.
Such a charge-coupled device is known from U.S. Pat. No. 4,280,068.
Charge-coupled devices with charge transport at a certain distance from the surface, so-called bulk transport, are well known and are designated by the abbreviations BCCD ("Buried Channel Charge-Coupled Device") and PCCD ("Peristaltic Charge-Coupled Device"). They are distinguished from analogue devices in which the charge transport takes place along the surface, inter alia, in that the transport efficiency is substantially not influenced by surface states. Moreover, in this device, the charge transport takes place under the influence of comparatively strong electrical fields because of the comparatively large distance between the charge packets and the transport electrodes. Devices of the BCCD type therefore generally operate at a very high speed.
It is desirable for most applications that the value of the charge packet to be formed under the input electrode varies practically linearly with the value of the input signal applied to the input electrode.
In charge-coupled devices of the BCCD type, however, potential wells are formed whose minimum is located at a certain distance from the semiconductor surface. With increasing value of the introduced charge packet, the stored charge always lies closer to the surface. The capacity of the charge storage space is therefore not constant, but increases with the value of the charge packet, so that upon conversion of the input signal into a charge packet a non-linearity is introduced. Therefore, attempts have been made to eliminate this undesired non-linearity in the input characteristic of such charge-coupled devices. For this purpose, it is suggested in the aforementioned U.S. Pat. No. 4,280,068 to generate under the input electrode a potential well whose minimum is located at a smaller distance from the surface than the minima of the potential wells generated under the transport electrodes. Due to the fact that it has been found that comparatively large quantities of charge can be stored closer to the surface without large capacity variations, in this manner a more linear input characteristic is obtained.
However, it has been found that also with the use of this measure in many cases the linearity of the input characteristic is nevertheless insufficient.